1. Field of the Invention
This invention relates to exposure trim mechanisms for photographic cameras and more particularly, it concerns improvements in the construction of such mechanisms by which both increased reliability in operation and reduced manufacturing costs are attained by comparison to the prior art relating to such trim mechanisms.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,209,243, issued June 24, 1980 to Bruce K. Johnson and George D. Whiteside discloses an exposure trim mechanism for photographic cameras of the type having an automatic exposure control system, including a scanning shutter, by which such exposure parameters the exposure interval and the effective flash aperture are dependent respectively on the amount of light passing from the scene to a photoresponsive detector and the range of the subject. For determining the effective flash aperture, an electronic strobe is fired at time periods (following initial shutter opening) which are selected in relation to subject range so that flash illumination is synchronized with a suitable aperture for each subject distance.
In the trim mechanism disclosed in the above-noted issued patent, for example, a manually adjustable slide is supported from the camera body both to regulate the speed to which a pair of variable aperture shutter blades are accelerated toward an open or exposure condition and to regulate the response characteristics of a photoelectric exposure control circuit in a manner to increase or decrease from a normal amount, the image light to which camera loaded film is exposed. Regulation of the shutter blade accelerating function, which primarily effects the flash contribution to the exposure, is achieved mechanically by adjustment of the starting point of an accelerator member. Photoresponsive regulation is achieved by a variable density filter carried by the slide in front of the photoresponsive detector or cell to vary the amount of subject or scene light passing to or "seen" by the cell. Both regulating functions contribute to adjustment of the film exposure. With respect to the variable density filter, for example, normal exposure of photographic film in the camera will result when a medial density portion of the filter is positioned in front of the cell. Correspondingly, the exposure control system may be trimmed to either an underexposure mode upon movement of the slide to reduce the filter density and thus increase the light seen by the cell to a value above that indicated for normal exposure, or to an overexposure mode by moving the slide in the opposite direction to increase filter density and thus reduce the amount of light seen by the cell to a value below that which would pass the filter for normal exposure.
On the other hand, the additional trim function provided by varying the rate of shutter blade opening acceleration is important where a burst of illumination, such as from a flash bulb or electronic flash, is used for film exposure. The additional trim function is needed to compensate for limitations on the overall shutter system to respond fully to measured light within the short time duration of this type of illumination. Thus, by slowing the initial shutter blade acceleration, the effective aperture reached by the shutter blades during at least the main portion of the flash is reduced. Correspondingly, the effective flash aperture can be increased from a norm by increasing the rate of shutter blade opening acceleration.
Although the exposure control systems and trim control mechanisms of the general type represented by the disclosure of the aforementioned U.S. patent are applicable to all types of photographic cameras, such systems and mechanisms have been used extensively in self-developing or instant cameras manufactured by Polaroid Corporation, Cambridge, Mass. Because of the facility offered by instant cameras for immediately retaking an improperly or undesirably exposed photograph, the trim mechanism has special utility in providing the camera user with an opportunity to correct or modify the darkness or lightness of the subject appearing in a photograph. The trim mechanism is equally important in taking a first photograph to photographers who recognize abnormal lighting conditions. For back-lighted subjects, for example, the trim mechanism should be set for a measure of overexposure (or lightening trim) because the back lighting will cause a corresponding measure of underexposure or darkening of the principal subject to be photographed. The opposite is true of a subject brightly illuminated by fore-lighting against a dark background.
The function of the trim mechanism disclosed in the aforementioned U.S. patent is, therefore, an important adjunct to the overall exposure control system for cameras generally and in particular, for the exposure control systems of self-developing or instant cameras of the type referred to. Because of the degree of precision needed for such regulation, the limitations on the space available for the overall exposure control system, and the importance of the trim mechanism to overall camera effectiveness, there is a need for an enhanced combination of structural simplicity, reliability and manufacturing cost effectiveness in trim mechanisms of this type.